The present invention relates to an electrochemical method and apparatus for electrochemically treating a solution or gas containing impurities or environmental pollutants, such as environmental hormones, to decompose the impurities into harmless low molecular weight compounds.
It has recently been recognized that atmospheric pollution and deterioration of the quality of water in rivers, lakes and marshes arising from industrial and livelihood wastes have adverse effects on the environment and human body. Thus, there is an urgent need to develop technical countermeasures against these problems. In the treatment of drinking water and the disposal of sewage and waste water, for example, chemicals such as chlorine have been added to decolor the water, lower COD or sterilize the water. However, the use of chlorine causes the production of new dangerous materials, namely, environmental hormones (exogenous secretion disturbing materials) and carcinogens. Thus, the use of chloride tends to be prohibited. In the incineration of wastes, carcinogen (dioxin) can be produced in the waste gas under some combustion conditions. Therefore, the safety of incineration has been questioned. In order to solve these problems, new methods have been studied.
In the electrolysis method, electric energy, which is a clean energy, is used to control chemical reaction on the surface of electrodes, thereby producing hydrogen, oxygen, ozone, hydrogen peroxide, etc. A substance to be treated can then be indirectly decomposed by these products. Alternatively, the substance to be treated can be adsorbed by the electrodes where it is then directly electrolyzed. Such an electrolysis has heretofore been employed to dispose of waste water. It is desirable that the decomposition products eventually become low molecular and safe materials such as carbon dioxide, water, hydrogen, oxygen, nitrogen, ammonia and chloride ion. However, it is known that some intermediate products in the course of decomposition are rather dangerous.
Extensive studies of electrodes and reactants are disclosed in xe2x80x9cDenki Kagakuxe2x80x9d, vol. 62, pp. 1,084-(1992), xe2x80x9cJournal of Applied Electrochemistryxe2x80x9d, vol. 21, pp. 99-104 (1991), etc. It is pointed out that substances to be treated cannot be sufficiently decomposed depending on the performance of electrodes. In general, anodization reaction by electrolysis of an aqueous solution results in the production of an electrolysis product with water as a starting material. In most cases, the oxidation of other ingredients cannot easily proceed in the presence of an electrode catalyst having a high reactivity with respect to the electric discharge of water.
Examples of electrode materials which perform oxidation include lead oxide, tin oxide, platinum, DSA and carbon. Examples of electrode materials which perform reduction include lead, iron, platinum, titanium and carbon. The material employable as an electrode substrate needs to be corrosion-resistant to prolong the cell life and prevent stain on the surface to be treated. The anodic power supplying material is limited to valve metals such as titanium and alloys thereof. The electrode catalyst is limited to noble metals such as platinum and iridium and oxides thereof. However, it is known that even these expensive materials, if used, are consumed and eluted with the electrolytic solution depending on the current density or time over which the cell is energized. It has thus been desired to provide electrodes having better corrosion resistance.
Graphite and amorphous carbon material have heretofore been used as electrode materials, but leave something to be desired in consumption resistance. In particular, these materials can be heavily consumed during positive polarization. Diamond is excellent in thermal conductivity, optical transmission and durability against heat and oxidation. In particular, The thermal conductivity of diamond can be controlled when properly doped with additives. Therefore, diamond has been favorably considered as semiconductor devices and energy conversion elements. Swain et al. reported the stability of diamond as an electrochemical electrode in an acidic electrolytic solution in xe2x80x9cJournal of Electrochemical Societyxe2x80x9d, vol. 141, pp. 3,382-(1994) and thus suggested that diamond is far superior to other carbon materials. Focusing on the magnitude of the band gap (4.5 V) of diamond, Fujishima et al. reported in xe2x80x9cJournal of Electroanalytical Chemistryxe2x80x9d, vol. 396, pp. 233-(1995) and xe2x80x9cDenki Kagakuxe2x80x9d, vol. 60, No. 7, pp. 659-(1992) that NOx can be reduced to ammonia using diamond. Some reports disclose a humidity sensor that makes the use of the change in surface resistivity of diamond with humidity [xe2x80x9cDenkironxe2x80x9d, vol. 114, No. 5, pp. 413-(1994)]. U.S. Pat. No. 5,399,247 suggests that the use of diamond as an anode material makes it possible to decompose an organic waste water.
Although sufficient reports have never been made on the industrial use of diamond electrodes at a high current density in a high potential range, it has recently been reported that diamond electrode is inert to the decomposition reaction of water and produces ozone and hydrogen peroxide besides oxygen in the oxidation reaction [Japanese Journal of Applied Physics, vol. 36, L260-, (1997)]. Hydrogen peroxide and ozone are materials from which OH radicals having a higher oxidizing power are produced. It is known that radicals can be easily produced in the presence of hydrogen peroxide or ozone. Accordingly, it is expected that the use of diamond as an electrode makes it possible to provide enhanced efficiency over systems using conventional electrodes. From a practical standpoint, improvements have been desired for further enhancement of cell efficiency.
It is therefore an object of the present invention to provide a method and apparatus which can attain further enhanced efficiency in electrochemical treating involving the decomposition of substances in waste water or the like using diamond as an electrode material.
The foregoing object of the present invention will become apparent from the following detailed description and examples.
The above objects of the present invention have been achieved by providing an electrochemical treating apparatus comprising an electrolytic cell comprising an anode and a cathode spaced apart from said anode, the anode including an electrode material made of diamond and the cathode including an electrode material made of diamond. The above objects of the present invention have also been achieved by providing an electrochemical treating method for electrochemically decomposing a substance contained in a gas or solution, which comprises introducing a gas or solution containing a substance to be treated into an electrolytic cell comprising an anode and a cathode spaced apart from said anode, said anode including an electrode material made of diamond and said cathode including an electrode material made of diamond, passing an electric current through the electrolytic cell, and recovering a treated gas or solution. In a preferred embodiment, the electrolytic cell comprises an anode including an electrode material made of diamond, a cathode including an electrode material made of diamond and an ion exchange resin or an ion exchange membrane as an electrolyte disposed between the anode and the cathode. A gas or solution containing a substance to be treated is contacted with the anode and cathode to decompose the substance into lower molecular weight components. Alternatively, a gas or solution containing a substance to be treated is contacted with the anode and cathode so as to generate oxidizing and/or reducing species which in turn act to decompose the substance into lower molecular weight components.